1. Field of the Invention
The present invention relates to a torque cam mechanism comprising first and second cam members which are disposed relatively rotatably around a common axis and cam balls adapted to fit in both first and second cam grooves formed in confronting surfaces of the first and second cam members which confront each other. In addition, the invention relates to a power transmission system for a four-wheel drive vehicle which utilizes the torque cam mechanism.
2. Description of the Related Art
The torque cam mechanism is known, for example, by JP-A-2001-213185.
In a torque cam mechanism of this type, when the first and second cam members rotate relatively in one direction, the cam balls ride on cam portions of the first and second cam members to thereby generate thrust forces in axial directions for separating both the cam members from each other, while when the first and second cam members rotate relatively in the other direction, the cam balls are locked at stopper portions in the first and second cam grooves to thereby restrain the relative rotation in the other direction.
Incidentally, when the first and second cam members do not rotate relatively, there should be generated no axial thrust force for separating both the cam members from each other. However, even though the first and second cam members do not rotate relatively but rotate together, when the cam balls are biased radially outwardly by a centrifugal force, there are generated thrust forces for separating the first and second cam members from each other. Consequently, in the event that this torque cam mechanism is provided in the multi-plate clutch, there is caused a problem that clutch plates are caused to engage by the thrust forces to thereby generate an undesirable drag torque.
The invention was made in view of the situation, and an object of the invention is to provide a torque cam mechanism which can reduce thrust forces for separating a pair of cam members from each other which are generated by a centrifugal force applied to cam balls.
With a view to attaining the object, according to a first aspect of the invention, there is proposed a torque cam mechanism comprising first and second cam members which are disposed relatively rotatably around a common axis and cam balls adapted to fit in both first and second cam grooves formed, respectively, in confronting surfaces of the first and second cam members which confront each other, wherein the first and second cam grooves are provided, respectively, with cam portions for permitting the movement of the cam ball in circumferential directions, whereby when the first and second cam members rotate relatively the cam balls ride on the cam portions of the first and second cam grooves to thereby generate axial thrust forces for separating both the confronting surfaces from each other, the torque cam mechanism being characterized in that the depth of one of the first and second cam grooves is larger than the radius of the cam ball, while the depth of the other is smaller than the radius of the cam ball.
According to the construction, since the depth of one of the first and second cam grooves is made to be larger than the radius of the cam ball, while the depth of the other is made to be smaller than the radius of the cam ball, even though the cam balls rotate together with the first and second cam members, a centrifugal force generated can be received by the deeper cam groove of the first and second cam members to thereby restrain the generation of thrust forces for separating the confronting surfaces of the first and second cam members from each other.
According to a second aspect of the invention, there is proposed a torque cam mechanism as set forth in the first aspect of the invention, wherein the first and second cam grooves are provided, respectively, with stopper portions for restraining the movement of the cam ball in a circumferential direction and cam portions for permitting the movement of the cam ball in circumferential directions, whereby when the first and second cam members rotate relatively in one direction the cam balls ride on the cam portions of the first and second cam grooves to thereby generate axial thrust forces for separating both the confronting surfaces from each other, while when the first and second cam members rotate relatively in the other direction the cam balls are locked at the stopper portions of the first and second cam grooves to thereby restrain the relative rotation in the other direction, and wherein the first cam member is provided with projections which project from its confronting surface in such a manner as to cooperate, respectively, with the stopper portions of the first cam grooves, while the second cam member is provided with projections which project from its confronting surface in such a manner as to cooperate, respectively, with the stopper portions of the second cam grooves.
According to the construction, since the projections are formed on the confronting surface of the first cam member in such a manner as to cooperate, respectively, with the stopper portions in the first cam grooves and the projections are formed on the confronting surface of the second cam member in such a manner as to cooperate, respectively, with the stoppers in the second cam grooves, when the first and second cam members rotate relatively in the other direction such that the cam balls are brought into abutment with the stopper portions in the first and second cam groove the occurrence of a case where the cam balls ride on the stopper portions in the first and second cam grooves can securely be prevented by the action of the projections on both the confronting surfaces.
According to a third aspect of the invention, there is proposed a power transmission system for a four-wheel drive vehicle which comprises a torque cam mechanism as set forth in the first or second aspect of the invention, the power transmission system for a four-wheel drive vehicle comprising front wheels to which the driving force of an engine is transmitted directly and rear wheels to which part of the driving force of the engine is transmitted indirectly via a multi-plate clutch, wherein the multi-plate clutch engages by virtue of hydraulic pressures generated by hydraulic pumps according to a difference in rotation between the front wheels and the rear wheels, as well as by thrust forces generated by the torque cam mechanism according to a difference in rotation between the front wheels and the rear wheels.
According to the construction, in the event that the rotating speed of the front wheels exceeds the rotating speed of the rear wheels when the vehicle runs forward, since thrust forces generated by the relative rotation of the first and second cam members of the torque cam mechanism immediately brings the multi-plate clutch into engagement, the response with which the vehicle is put in a four-wheel drive condition can be increased by transmitting the driving force from the front wheels to the rear wheels immediately the front wheels slip to generate the difference in rotation between the front and rear wheels when the vehicle starts to move forward from a standstill or is drastically accelerated while moving forward. Thereafter, since the multi-plate clutch is also brought into engagement by hydraulic pressures generated by the hydraulic pumps with a slight time-lag, a sufficient amount of driving force that is transmitted from the front wheels to the rear wheels can be secured. In contrast, in the event that the rotating speed of the rear wheels exceeds the rotating speed of the front wheels by panic braking when the vehicle is moving forward, since the torque cam mechanism generates no thrust force, any interference with the ABS mechanism can be avoided by preventing the transmission of the driving force from the front wheels to the rear wheels. Moreover, since the generation of thrust forces for separating the first and second cam members from each other by the centrifugal force applied to the cam balls can be restrained to a minimum level by the torque cam mechanism in which the depths of the first and second cam grooves are different, whereby the generation of unwanted drag torque in the multi-plate clutch can be prevented. Thus, the construction can contribute to the reduction in running resistance, as well as fuel consumption.